The overall objective of this project is to understand the cellular and molecular mechanisms responsible for the specification, patterning, and differentiation of internal organs during development. More specifically, how the elaborate network of blood vessels arises during vertebrate embryogenesis. Blood vessels are ubiquitous and vital components of all vertebrate animals, innervating and supplying every tissue and organ with oxygen, nutrients, and cellular and humoral factors. They have also become a subject of great clinical interest in recent years, particularly with the potential shown by antiangiogenic therapies for combating cancer. Many of of the recent insights into mechanisms of blood vessel formation have come from developmental studies including those being performed as part of this project. The zebrafish, a small tropical freshwater fish, possesses a unique combination of features that make it particularly well suited for studying blood vessels. The fish is a genetically tractable vertebrate with a physically accessible, optically clear embryo. These features provide a tremendous advantage for studying vascular development- they permit observation of every vessel in a living animal and simple, rapid screening for even subtle vascular-specific mutants. Major aims of the laboratory include: (i) Developing experimental tools for experimental analysis of vascular development in the zebrafish, (ii) Genetic analysis of vascular development, (iii) Molecular dissection of arterial-venous development, (iv) Analysis of vascular patterning and morphogenesis Developing Tools for Experimental Analysis of Vascular Development in the Zebrafish An important aim of this project has been to develop new experimental tools for studying blood vessel formation in this organism. To this end, we previously established a microangiographic method for imaging patent blood vessels in the zebrafish and used this method to compile a comprehensive staged atlas of the vascular anatomy of the developing fish (http://eclipse.nichd.nih.gov/nichd/lmg/redirect.html). We also generated transgenic zebrafish lines expressing green fluorescent protein (GFP) in vascular endothelial cells (VEC), making it possible for us to visualize the blood vessel formation in intact, living embryos. We have developed methodologies for long-term multiphoton confocal timelapse imaging of the dynamics of blood vessel formation in these transgenic fish, and we have used these methods to examine the morphogenesis of developing trunk and cranial vessels and adult fin blood vessels. An important current priority in the laboratory is the development of additional transgenic lines that permit dynamic visualization of specific subsets of vessels such as arteries and veins, or subcellular structures within vascular cells (see below), or that permit temporally regulated gene expression within the vasculature. Genetic analysis of vascular development Genetic dissection of vascular development and the molecular pathways that regulate it is an important ongoing goal of the UVO. We employ unbiased, forward genetic mutational screening approaches to identify and characterize zebrafish mutants that affect the formation of the developing vasculature. We have already positionally cloned the defective genes from a number of vascular patterning mutants, including violet beauregarde (defective in Alk1/acvrl1), y10 (defective in phospholipase C-gamma 1), and kurzschluss (defective in a novel chaperonin). We are currently carrying out an ongoing large-scale genetic screen for additional ENU-induced mutants using transgenic zebrafish expressing green fluorescent protein (GPF) in blood vessels. We have screened well over 2000 genomes to date, and identified approximately 80 new vascular mutants with phenotypes including loss of most vessels or subsets of vessels, increased sprouting/branching, and vessel mispatterning. A bulked segregant mapping pipeline is in place torapidly determine the rough position of newly identified mutants on the zebrafish genetic map, and fine mapping and molecular cloning is in progress for many mutants. These ongoing mutant screens should \yield a rich harvest of novel vascular-specific mutants and bring to light new pathways regulating the specification, differentiation, and patterning of the developing vertebrate vasculature. Molecular Dissection of Arterial-Venous Development We have previously uncovered a molecular pathway regulating the acquisition of arterial-venous identity consisting of sonic hedgehog (SHH), vascular endothelial growth factor (VEGF), phospholipase C gamma-1 (a major downstream effector of VEGF signaling), and notch signaling acting in series. Subsequent studies from other labs have supported a similar role for this novel pathway during mammalian arterial differentiation. We our continuing efforts to dissect arterial-venous development using genetic screens, microarrays, and novel transgenic tools. Our ongoing mutant screens have identified several new loci that are required for either arterial or venous vascular development, and cloning and molecular characterization of these new mutants is in progress. We have established microarray technology in the laboratory, and are screening animals with gain- or loss- of function for each of the steps in our previously characterized arterial differentiation pathway. Using our microarray screen we have identified approxinately 150 differenbtially expressed genes potentially involved in the A-V differnetiation pathway, and are in the process of carrying out expression and functional analysis of these candidates. Finally, we are establishing arterial- and venous-specific transgenic lines that will allow us to easily assay for arterial and venous differentiation states in vivo. Analysis of Vascular Patterning and Morphogenesis An important research area has been studying the mechanisms and molecular basis for vascular patterning and morphogenesis during development, in particular how vascular networks assemble with a defined, stereotypic anatomical pattern. We have used multiphoton time-lapse imaging of the developing trunk to obtain important new insights into how angiogenic networks assemble in vivo. These insights include elucidation of a novel 2-step model for vascular network formation and evidence for genetic determination of vascular pattern. Additional studies have allowed us to define some of these genetic pattern determination factors, showing that well-known neuronal guidance factors also play an important role in vascular guidance and vascular patterning. We have recently found that Semaphorin-Plexin signaling it is an essential determinant of trunk vessel pattern, uncovering a Plexin gene expressed specifically in the vasculature. In the developing trunk, angiogenic intersegmental vessels extend near somite boundaries. Loss of Plexin function via morpholino injection or genetic mutation causes dramatic mispatterning of these vessels, which are no longer restricted to growth near intersomitic boundaries. Somites flanking intersegmental vessels express Semaphorins and reducing the function of these Semaphorins also causes intersegmental vessel mispatterning. These results indicate that the establishment of anatomical pattern in the developing vasculature is directed in part by cues and mechanisms similar to those used to pattern the developing nervous system, including Semaphorin-Plexin signaling. We are currently attempting to extend these studies and uncover the signal transduction machinery that transmits Semaphorin-Plexin signaling. We have already identified a number of potentially interacting genes using 2-hybrid screening, and have uncovered new loci that influence vascular patterning in vivo using genetic screening.